1. Kinetics and Mechanism of Xenobiotic Degradation Induced by Dioxygen Activation
Christina Noradoun
University of Idaho
Chemistry Department
Moscow, ID

2. Outline Introduction General Reaction Scheme
Environmental Problem
The Importance of Dioxygen Activation
Biological vs. Chemical Activation
General Reaction Scheme
Degradation Kinetics and Reaction Products
Xenobiotic: Environmental Pollutants
Chlorinated phenols
Organophosphorus and nitrated compounds
EDTA
Mechanism of Degradation
Conclusions

3. Research Problem
The disposal of organic pollutants and common chemical warfare agents is a matter of increasing concern.
The 1997 Chemical Weapons Convention Treaty mandated the eradication of all chemical weapons by the year 2007, later extended to 2012.
As of November 2003 only 11% of the 70,000 metric tons of chemical weapons stored worldwide had been destroyed.
US and Russia are holding 95% of all stockpiles and are unlikely to meet 2012 deadline.
Lapses in arms disclosures and delays in chemical weapons destruction prompt proliferation fears
Pifer, A.; et.al. J. Am. Chem. Soc. 1999, 121, C&EN News; May 5, 2004

4. Chlorinated Pollutants
PCBs (polycholorinated biphenyls)
290 million pounds are located in landfills and storage facilities in the USA
PCP (Pentachlorophenol, wood preservation)
Pesticides
Aldrin/Dieldrin (termiticide, banned 1974)
Chlordane (EPA banned sales 1988)
DDT (EPA banned all public uses 1972)l
Heptachlor (banned 1983)
Hexptachlorobenzene (banned 1984)
Agency for Toxic Substance and Disease Registry;

5. Incineration: Up in Smoke
The only approved EPA method for stored nerve agents and most common method for chlorinated waste removal
Economically prohibitive (3000 ºC)
Source of polycylic aromatic hydrocarbons (PAHs), such as chlorinated or brominated dioxins
Disposal concerns of the tons of toxic bottom and fly ash.
TRANSPORTATION
U.S. Army operated incineration plant in Anniston, AL
Ember, Louis; C&EN News; 2004, 82, 25. Wang, Lin-Chi; et.al; ES&T, 2002, 36, Soderstrom, Gunilla; et. al; ES&T. 2002, 36,

6. Dioxins
The term “dioxin” signifies the family of polychlorinated dibenzo-p-dioxins and furans
The most toxic subgroup is chlorinated in the 2,3,7,8 positions (ex. 2,3,7,8-tetrachloro-benzo-p-dioxin)
These compounds can form in combustion of chlorine-containing organic materials
C&EN; Oct 8, 2004, 82, 40.

7. Alternative Available Methods
This 1-ton tank contains aging mustard gas that will be destroyed at Tooele Army Facility in Utah, beginning of summer 2005.
Biological Oxidation
Often incomplete
short catalytic lifetime
Not applicable to high pollutant concentrations
thermal sensitivity
inexpensive resources and produce little byproducts.
Ember, Lois; C&EN News, 2004, 82, 8. Wang, Lin-Chi; et.al; ES&T, 2002, 36, Soderstrom, Gunilla; et. al; ES&T. 2002, 36, , Fighting Nerve Agent Chemical Weapons with Enzyme Technology; Annals of the New York Academy of Sciences 864: (1998) .

8. Chemical Oxidation
These chemical oxidation techniques are successful at oxidizing nerve agents although there are drawbacks
Peroxygen Oxidizers
perborate, peracetic acid, m-chloroperoxybenzoic acid
In complete oxidation
Caustic Bleaching Agents
In 1917 the Germans used bleaching powder to neutralize mustard agent.
Requires solubilization therefore large quantities of solvent that must be dealt with
Supercritical H2O
Material and energy costs are large when considering any large-scale demilitarization processes
Annals of the New York Academy of Sciences 864: (1998), Formulations for the Decontamination of CB Warfare Agents; Annual Report MOD M February 2001; Sandia National Lab.

9. Overall Goal
The destruction or neutralization of xenobiotics, including nerve agents and chlorinated pesticides using green oxidation chemistry.
Produce a low cost alternative to incineration by working at Room Temperature and Pressure Conditions (RTP)
Common Reagents with Long Term Storage
Focus on non-biological oxygen activation to eliminate the need for specialized catalysts

10. Molecular Oxygen
How does one tap into the seemingly stable energy source?
Oxygen’s two unpaired electrons make it difficult to accept a bonding pair, hence the reluctance to react by forming new chemical bonds
Two ways of overcome this
Oxygen can absorb energy from other molecules that have been excited by heat or light
Add electrons to oxygen one at a time
Iron which also has an unpaired electron is efficient at donating electrons to oxygen

11. Molecular Oxygen as an Oxidant
Most attractive oxidant for green oxidations is O2 from air.
Hydroxyl radical can react within billiseconds
Diagram showing reaction oxygen intermediates between O2 and H2O. Hydrogen left out for simplicity

12. Hydroxyl Radical and The Fenton Reaction
H2O2 + e-  HO• + HO-
Fe(II)  Fe(III) + e-
Fe(II) + H2O2  Fe(III) + HO• + HO-
The impact of Ferrous salts on H2O2 reduction was discovered over 100 years ago by Henry Fenton.5
The Fenton reaction in form above, including the hydroxyl radical, was suggested over 75 years ago.6
H.J.H. Fenton. J. Chem. Soc. 1894, 65, 889.
F. Haber and J.J. Weiss. Proc. Roy. Soc. London, Ser. A , 147, 332.

13. Oxygen Activation Biological Chemical
cytochrome P450 enzymes, monooxygenase
Chemical
GIF reaction
TAML ligand - hydrogen peroxide activator

14. Cytochrome P450 enzymes: Nature’s Oxidative Workhorse
•Large family of enzymes
•Catalyzes redox-processes
•The major system for drug and xenobiotic metabolism.
•Highest concentration in the liver.
•The CO complex absorbs at 450 nm.
•Active center: Protoporphyrin + Fe(III) + Cys.

15. Catalytic cycle for cytochrome P450 monooxygenations
From an inorganic chemist's perspective, P450 enzymes are fascinating due to their ability to activate molecular oxygen to react with organic substrates with a selectivity and efficiency unparalleled in synthetic systems.
Chem. Soc. Rev., 2000, 29, 375–384

16. Non-biological oxygen activation
The pioneering research focused on iron porphyrins as biological mimics.
Barton and co-workers developed a non-porphyrin iron catalyst system that has come to be known as the “Gif system”.
Main thrust of this research was centered around highly selective oxygenation for industrial synthesis.
Protoporphyrin-IX

17. RH + O2 (2 H+, 2e-, Mn+)  ROH + H2O
General Gif Reaction
Gif reactions are capable of catalyzing monooxgenation of carbon-hydrogen bonds, to produce ketones.
 RH + O2 (2 H+, 2e-, Mn+)  ROH + H2O
General requirements: reducing agent (electron source), protons, a catalytically active metal ion (Fe2+, Cu2+), oxygen and a solvent.
The major disadvantage of Gif-type reactions for environmental remediation is the expense and toxicity of the necessary solvent pyridine.
Barton, D.H.R; Doller, D. Acc. Chem. Res , 25, ; Stravropoulos, P.; Celenligil-Cetin, R.; Tapper, A.; Acc. Chem. Res. 2001,34,

18. Hydrogen Peroxide Activators
Dr. Terrence Collins at the Institute of Green Oxidation Chemistry has pursued the design and synthesis of hydrogen peroxide activating catalyst for the past 20 years.
TAML ligands can activate H2O2 to strong oxidizing agents capable of breaking down pollutants in aqueous and non-aqueous solutions.
TAML ligands are uniquely designed to be inert to internal ligand oxidation.
Add a oxidation number to the ligand
Collins, T.; Acc. Chem. Res. 2002,35, Collins, T.; Acc. Chem. Res. 1994, 27,

19. TAML Degradation
9 minutes, 99% degradation of TCP (2,4,6-trichlorophenol)
TAML/TCP ratios--1:2000
H2O2/TCP ratios—100:1
pH 10 and 25C
Add TCP structure
Gupta, S.; Stadler, M.; Noser, C.; Ghosh, A.; Steinhoff, B.; Dieter, L.; Horwitz, C.; Schramm, K.; Collins,T.; Science, 2002, 296,

20. Review of current oxygen activation systems
Biological oxygen activation, P450 enzymes
Gif and TAML the major drawbacks are the requirement of expensive reagents and incomplete degradation
The proposed system uses only zero valent iron, EDTA and air
Is the only system know to date that can take O2 and convert it to potent oxidizing species capable of extensively degrading xenobitics

21. Reaction Scheme: Fe°, EDTA, Air
releases Fe2+
site for reduction FeIIIEDTA
EDTA
promotes Fenton reaction
promotes FeII solubility
enhance dissolution of Fe2+

22. General reaction conditions for Xenobiotic degradation
0.5g Fe; mesh
0.44mM Xenobiotic
10.0 mL water
Air flow
2.0 mL 50/50 hexane/ethyl acetate
(extraction only)
Stir bar
0.44mM EDTA
One reaction vessel was generated for each data point.
Degradation curves represent 8-15 individual reaction vials each extracted and analyzed using GC-FID or HPLC.
@ 25°C, pH
Noradoun, C; et.al. Ind. & Eng. Chem. Res , 42(21),

23. Phenol Degradation Using: Iron metal, EDTA, and air
Results have shown >90% degradation of 1.26 x 10-3 M phenol in under 3 hours.
Pseudo-First Order
Rate constant: /M hr
Direct aqueous injection using HPLC
HPLC Mobile Phase: 60/40 water/ methanol (1% Hac) Flow rate (1ml/min)
UV: 270nm , C18 column

24. 4-chlorophenol (4CP) Degradation
Results have shown >95% degradation of 1mM 4-chlorophenol (4CP) in under 4 hours. (hexane/ethyl acetate extraction GC-FID)
Pseudo-first order rate constant: /M hr @ 25°C, pH 5.5
Noradoun, C; et.al. Ind. & Eng. Chem. Res , 42(21),

25. ESI-MS of 4CP after 4 hours of degradation
None of the degradation products were detected in the organic extractions (GC-FID) or direct aqueous injections (HPLC), therefore ESI-MS was conducted.
M-1 ion peaks. Results show the complete degradation of 4CP (m/z 127), as well as the absence of any chlorinated products.
FeIIIEDTA
Iminodiacetic Acid
HCO3-
propionic acid
oxalate
succinic acid
Noradoun, C; et.al. Ind. & Eng. Chem. Res , 42(21),

26. 4-chlorophenol (4CP) Degradation cont.
0.5 g of mesh Fe°
10 mL of solution
1mM EDTA
1mM 4-chlorophenol
4 hour reaction time
GC-FID and ESI-MS
RTP

27. Summary of ESI-MS analysis of 4CP reaction
Complete destruction of 4CP after 4 hours
No chlorinated products produced during any time of the reaction (1hr-4hrs).
Ring opening produces low molecular weight acids, succinic, oxalic etc.
No evidence of Cl- was found in any of the ESI-MS, even when NaCl was spiked into the sample. The chloride is thought to be adsorbed to the iron surface.

28. Organophosphorous Nerve Agents and Nitrated Explosive Surrogates
TNT surrogate, nitrobenzene (985 ppm) was decomposed in 24 hours.
VX surrogate, malathion (49 ppm) was consumed in 4 hours, to give diethyl succinate. Malathion was the only pollutant to give a by-product detectable by GC-FID.

29. Malathion Degradation
PO43- + SO42-
malathion
DES
malaoxon
max: 4-6 hrs
Max: 7 hrs
SO42- :0.0593mM (14% yield) (24hrs)
PO42- : mM (19 % yield) (24hrs)

30. Kinetics of Malathion Degradation
Diethyl Succinate (DES)
****Do a carbon balance using excel.
GC/FID chromatograph: each data point indicates an individual reaction vial extracted using 50/50 hexane/ethyl acetate, error bars indicate the standard deviation between three measurements of each sample vial.

31. ESI-MS Time: 0 hrs Time: 12 hrs Reaction Conditions 0.44mM Malathion
EDTA
Malathion (m/z 329)
Iminodiacetic Acid
HCO3-
oxalate
propionic acid
Malaoxon (m/z 315)
Reaction Conditions
0.44mM Malathion
0.44mM EDTA
0.5g FeO Air

32. EDTA an Environmental Concern
Control studies show EDTA degradation as well as xenobiotic degradation.
Why is EDTA an environmental concern?
Used as a metal chelation agent in a wide variety of applications including:
Paper-pulp bleaching
Photochemical processing
Lumber industry
Cosmetics, Detergents
Currently not being monitored or treated at waste water treatment facilities
Concern for heavy metal mobility and longer bioavailability of metals to aquatic plants and animals
Stable in aquatic environment
Anthropogenic
Structures

33. Experimental Setup 1 mM EDTA (Total Vol. 50mL) 2.5g Fe°
Open to the Atmosphere
Aliquots were taken directly from reaction vessel, diluted, filtered and injected into HPLC
125 ml round bottom flask
stir bar
2.5 g Fe°
BAS stir plate

34. HPLC conditions for FeIIIEDTA detection
EDTA non-extractable using organic solvent must use direct aqueous injection
EDTA alone not absorb, however FeIIIEDTA complex does at 258nm
Mobile phase: 0.02M formate buffer, pH 3.3
Containing: TBA-Br (0.001M) and acetonitrile (8%)
Flow rate: 1ml/min
Temp: ambient temp
UV = 258 nm
Sample volume 20µL
Column RP-C18 +
TBA-Br
Nowack et. al.; Anal. Chem. 1996, 68, 561

35. EDTA degradation
Pseudo-first order plot showing linearity for EDTA degradation from 10min-2.5hrs.
kobs = /M hr
1mM EDTA, 2.5 g Fe° and air (▲), control in the absence of iron (■)

36. ESI-MS
background
No Fe°, N2
1mM FeSO4 1mM EDTA
4hrs
Fe°, Air
1mM EDTA

37. Kinetically stable organic species in the presence of aqueous Fe(0)/EDTA/O2
iminodiacetic acid
succinic acid
bicarbonate
propionic acid
oxalate
Degradation products for -EDTA
-Malathion
-4-chlorophenol
-pentachlorophenol
-phenol

38. A More in Depth Investigation…
Longevity
Understanding Reaction Mechanism
Reactive Oxygen Intermediate species
Reaction Kinetics
Optimization of Experimental Parameters

39. Prolonged Degradation of EDTA
ZVI maintains EDTA degradation without significant loss in the observed rate over a time period of several hours
All systems mixed at 450 rpm, open to atmosphere, unbuffered using 2.5g ZVI.

40. Reactive Oxygen Species
ROS
O2-•, OH-, FeV=O, etc.
Two Analyses were performed
Thiobarbituric acid-reactive substances (TBARS) assay
Addition of known radical scanvenger, 1-butanol

41. Thiobarbituric acid reactive substances assay (TBARS)
HO·, FeIV=O
Malonaldehyde bis(dimethyl acetal)
TBA
Deoxyribose
534 nm
Nonselective detection of reactive oxygen species oxidizing species.
Junqueira VB; Mol Aspects Med Feb-Apr;25(1-2): Hader D; Photochem Photobiol Sci Oct;1(10):

42. Noradoun, C; et.al. Ind. & Eng. Chem. Res. 2003, 42(21), 5024-5030.
TBARS cont.
Results of HO· radical trapping by deoxyribose/thiobarbituric acid system forming a chromgen (534 nm). The conditions were 30 minutes of reaction time with 0.10 g mesh Fe(0), under aerobic conditions.
Absorbance Units at
534 nm
Control 1 – 0 mM deoxyribose, 2.39 mM EDTA
0.0
Control 2 – 3.18 mM deoxyribose, 0 mM EDTA, - also N2 flow, -No Fe(0)
0.149
3.18 mM deoxyribose, 2.39 mM EDTA
0.846
Noradoun, C; et.al. Ind. & Eng. Chem. Res , 42(21),

43. Suppression of EDTA degradation with the addition of Radical Scavenger
Alcohols such as 1-butanol are known to be •OH radical scavengers
(■) kobs = M-1hr-1
(▲)kobs = M-1hr-1 with 5mM 1-butanol
(2.5 g ZVI g, 1.00mM EDTA, open to air)
Mantzavinos D; Water Res Jul;38(13): J Hazard Mater Apr 30;108(1-2):

44. TBARS assay indicates reactive oxygen species are present
While 1-butanol studies indicate that •OH radicals are an important part of the reaction mechanism.
Further studies using the newly acquired departmental ESR, would give insight as to the specific type of radical species present.

45. Kinetic Parameters Examined
Future industrial scale up would require the knowledge of how do these parameters effect the observed reaction rate?
EDTA concentration
Fe° mass (surface area)
Rate of mixing
Temperature
A better understanding of the rate-limiting step in the reaction sequence could allow one to possibly speed up the reaction.

46. Rate limiting step Fe° → Fe2+ (dissolution)
Fe2+ → FeIIEDTA (Fe-EDTA formation)
Homogenous chemical steps
FeIIEDTA + O2 ↔ FeIIEDTA-O2
FeIIEDTA-O2 → FeIIIEDTA + O2•-
O2•- + O2•- + H+ → H2O2
FeIIEDTA + H2O2→ FeIIIEDTA + HO• +HO- (Fenton rxn)
Xenobiotic degradation
Heterogeneous reduction steps
FeIIIEDTA  FeIIEDTA

47. How does Fe-oxide layer and adsorbed EDTA effect rate?
Experimental setup:
Hold Fe° mass constant and vary concentration of EDTA and measure the observed rate constant

48. EDTA and other dicarboxylic acids enhance dissolution by shifting electron density towards the metal ion and simultaneously enhancing surface protonation therefore weakening the Fe-oxygen lattice bonds.
Stumm, W; “Chemistry of the Solid-Water Interface”; John Wiley & Sons, Inc. NY, © 1992, p204

49. EDTA degradation rate effected by EDTA concentration
Theory would suggest [Fe2+] released should be proportional to [EDTA], therefore more [EDTA] should enhance degradation rates. If degradation rates are based upon Fe dissolution rates.
The opposite was found experimentally.
Important point: Simply adding more EDTA will not speed up reaction.
2.5 g Fe°, open to atmosphere, 450 rpm, total rxn volume 50mL

50. Possible mechanisms for suppression of the reaction by excess EDTA
Surface Controlled
EDTA hindering dissolution at high concentrations
Reduction of FeII/III at the iron surface inhibited by excess EDTA
Non-Surface controlled
Fenton Chemistry: High FeII/III:EDTA ratios in solution has been shown to inhibit Fenton reactivity*.
*Engelmann, M; et.al. Biometals, 2003, 16, 519.

51. EDTA hindering dissolution at high concentrations
Measurement of the dissolution rate was done using an electrochemical cell designed specially to measure corrosion rates at metal surfaces
Experimental design
Varying EDTA concentration, while maintaining constant Fe° mass (surface area).

52. Corrosion Cell
Working Electrode: Fe° (99%), 3/8" diameter by 1/2" length (surface area 5.22 x 10-4 m2)
Counter Electrode: high density graphite rod
Reference: Standard Calomel Electrode (SCE), glass luggin capillary
1 liter glass cell
Polished working electrode with 600 grit sandpaper between sample runs
Used 50mM KNO3 as electrolyte in all samples

53. Tafel Corrosion Analysis
Corrosion normally occurs at a rate determined by an equilibrium between opposing electrochemical reactions.
Anodic reaction: metal oxidized, releasing electrons into the metal.
Fe° Fe2+ + 2e-
Cathodic reaction: solution species (often O2 or H+) reduced, removing electrons from the metal.
2H+ + 2e-  H2

54. Corrosion Rate
Addition of EDTA does enhance dissolution rates to a certain point (~5mM)
Overall corrosion rates for in the presence of N2 are higher than air
Passivation layer forming on the Fe° surface in the presence of O2 in air
Important point is the dissolution is not hindered by excess EDTA
Rate-limiting step is not Fe° dissolution
N2 purge
Air purge

55. If EDTA does not hinder the dissolution, what causes the reaction rate to decrease?
1. Surface chemistry : Reduction of FeII/III at the iron surface inhibited by excess EDTA
2. Solution chemistry: High FeII/III:EDTA ratios inhibiting Fenton reactivity.

56. High FeII/III:EDTA ratios inhibit Fenton reactivity
Previous work by earlier groups members has shown cases of ratios of FeII/III:EDTA more than 1:3 in which Fenton reactivity is hindered
Which explains the duality of EDTA acting as both a pro-oxidant and an antioxidant.
It was shown that Ca2+ metal could be added to sequester the excess EDTA.
Fenton reactivity was then shown to return due to the return of the optimal values of FeII/III:EDTA (1:1).
The exact coordination chemistry of FeII/III:EDTA in aqueous solutions remains uncertain
*Engelmann, M; et.al. Biometals, 2003, 16, 519.

57. Calcium Addition
The addition of 10mM Ca2+ did not effect degradation rate.
10mM EDTA, 2.5g Fe °
kobs = M-1h-1 (with Ca2+)
kobs = M-1 h-1
kobs = M-1 h-1
1mM EDTA, 2.5g Fe °
kobs = M-1h-1
2.5 g Fe°,open to air, total rxn volume 50mL

58. Calcium Addition cont.
Ca2+ addition had no overall effect on the rate of degradation
The added Ca2+ also did not help sequester excess EDTA in solution
Therefore there was no improvement of Fenton Reactivity with the Ca2+ addition
Alternative way of examining the problem was to hold EDTA concentration constant and vary amount of Fe° present

59. Role of Fe° mass/surface area in observed rate constant
surface area, kobs (0.29 m2, /Mh)
Increased levels of Fe°, enhance the rate of degradation by maintaining a balance between the Fe2+ and [EDTA]
(0.028 m2, /Mh)
0.10g g Fe°, 1.00mM EDTA, open to atmosphere, 450 rpm, total rxn volume 50mL
BET surface area analysis m2/g : Porous Material Inc., Ithaca, NY

60. Maintaining proper Fe°to EDTA ratios
Interactions between EDTA and Fe2+ are important factor controlling the degradation rates
Due to the duality of EDTA acting as both a pro-oxidant and antioxidant, controlling the [EDTA] is imperative to the success of the process.
Rate-limiting step
1. Surface chemistry : Reduction of FeII/III at the iron surface inhibited by excess EDTA
2. Solution chemistry: High FeII/III:EDTA ratios inhibiting Fenton reactivity.

61. General Model Mass Transport-limited Kinetics
mass transport of FeIIIEDTA to the Fe° surface
FeIIIEDTA + e-  FeIIEDTA
mass transport of FeIIEDTA to the bulk soln.
“A common criterion for detecting mass transport-limited kinetics is variation in reaction rate with intensity of mixing. Rates that are controlled by chemical reaction step should not be affected, where as aggressive mixing usually accelerates diffusion-controlled rates by reducing the thickness of the diffusion layer.”
Leah Matheson and Paul Tratnyek; ES&T. 1994,

62. Effect of mixing rate on observed degradation rate constant for EDTA
Good indication that rate-limiting step of EDTA degradation involves mass transport and not chemical reactions occurring in the bulk solution
2.5 g Fe° g, 1.00mM EDTA, open to air, total rxn volume 50mL

63. If reaction is mass transport controlled rate limiting step likely:
FeII/IIIEDTA reduction at iron surface
Can not rule out the heterogeneous O2 activation
Mass transport of oxygen from the bulk solution to the reacting iron surface is enhanced by the fluid flow.
Typical bulk oxygen concentrations at room temperature in aqueous solutions are 0.25mM (8ppm).

64. Temperature Experiments
The last kinetic parameter investigated was the effect of temperature on the reaction mixture
Temperature was varied using a temperature bath and a jacketed water cell
An Arrhenius plot was constructed to obtain the activation energy

65. Arrhenius plot shows dependence of observed rate constants on temperature
k = A exp(-Ea/RT)
2.5g Fe°, 1mM EDTA, 50ml total volume, reactions conducted using a temperature bath and a water-jacketed cell

66. Comparison Studies
Auto-oxidation of FeII to FeIII by O2 in aqueous solutions
Significantly enhanced by EDTA
FeII:EDTA ratios were important
1:1 ratios were reported as optimal
1:20 ratios showed a significant decrease in the autoxidation process
R. Van Eldik; Inorg. Chem.; 1990, 29, (* 0.02M [Fe(EDTA)])

67. Similiar studies show the rate limiting step as
FeIIEDTA + O2  FeIIEDTAO2 k1 = 107/Ms
FeIIEDTAO2  FeIIIEDTA + O2- k2 = 102/Ms
FeIIEDTAO2 + H+  FeIIIEDTA + HO2 k3 = 1010/Ms
Rate limiting step is the activation of oxygen at the iron coordination site
Activation energy of 33.9 kJ/mol*
Similiar studies show the rate limiting step as
FeIIEDTA + O2 + H+  FeIIIEDTA + H2O2
Activation energy of 27.2 kJ/mol**
R. Van Eldik; Inorg. Chem.; 1990, 29, (* 0.02M [Fe(EDTA)],pH=5), Beenackers, A.; Ing. Eng. Chem. Res. 1992, 32, 2580.(**[EDTA]=100mol/m3 pH=7.5)

68. Rate limiting step Fe° → Fe2+ (dissolution)
Fe2+ → FeIIEDTA (Fe-EDTA formation)
Homogenous chemical steps
FeIIEDTA + O2 ↔ FeIIEDTAO2
FeIIEDTAO2 → FeIIIEDTA + O2•-
O2•- + O2•- + H+ → H2O2
FeIIEDTA + H2O2→ FeIIIEDTA + HO• +HO- (Fenton rxn)
Xenobiotic degradation
Heterogeneous reduction steps
FeIIIEDTA + e-  FeIIEDTA
Potential rate limiting step with an activation energy of 25 kJ/mol
Can’t rule out heterogeneous rate limiting step with mass transport limited kinetics

69. Conclusions
Take home message: This system is a viable option for environmentally remediation of a variety of pollutants and has a strong possibility for scale up.
The only system known to date that can obtain non biological Oxygen Activation at room temperature and pressure to produce reactive oxygen species that are capable of fully degrading pollutants
Due to the duality of EDTA acting as both a pro-oxidant and antioxidant, controlling the [EDTA] is imperative to the success of the process.
Rate-limiting steps are controlled by oxygen activation and transport characteristics

70. Acknowledgments Dr. Frank Cheng Cheng Group
Dr. Malcolm and Mrs. Renfrew
Synder and Renfrew Scholarships
National Science Foundation